JENNIFER Lauren, the niece of designer Ralph Lauren, has been fined over a foul-mouthed rant aboard a plane that had to be diverted to Ireland.

Lauren pleaded guilty to breaching the peace and being drunk on board a transatlantic Delta Air Lines flight which had to be diverted to Shannon Airport in the west of Ireland on Monday afternoon.

The 41-year-old appeared before Ennis District Court in Co Clare, where her solicitor claimed Lauren was upset and embarrassed by her actions, which she said were out of character and caused by a reaction to alcohol mixed with medication.

More than 200 passengers and crew were on board the flight from Barcelona to New York when the air rage incident took place.

In the spotlight ... Jennifer Lauren's solicitor said her air rage was out of character. P...Source:AP

The diversion cost Delta $US43,000 ($A48,500).

Lauren bit her lip and looked to the ground in the dock while Garda Inspector Tom Kennedy told the court she had been loudmouthed, abusive and threatening on the aircraft.

He said the flight, bound for New York's JFK airport, had been in the air for two hours when an air hostess noticed the defendant crying in her seat, which would not recline properly.

He said Lauren told stewardess Constance Topping to "get the f--- out of my face" as she tried to help, and when told to calm down she became more abusive.

The court heard Topping went to brief her supervisor Jennifer Simpson at the top of the plane and Lauren followed her through first class and in the galley "at speed" where she ranted, roared and shouted incoherently.